Field of the Invention
This invention relates generally to the field of acoustic digital signal processing and the use of multi-microphone signal processing algorithms.
Description of Related Art
The use of multi-microphone signal processing algorithms to improve speech quality is pervasive in portable audio products such as mobile phones and tablets.
Beamforming algorithms utilize two or more microphones to selectively enhance a wanted signal, such as speech, and suppress unwanted noise. FIG. 1 shows an example of a possible implementation of such a system on a Digital Signal Processor (DSP) 100. This example consists of a set of analog microphones connected to Analog to Digital Converter (ADC) 110 of a DSP chip, which routes the digital signals to beamforming algorithm 120. This algorithm produces processed output, which can be routed for further processing within the DSP or output, digitally or as an analog signal via a Digital to Analog Converter (DAC). An implementation in silicon is also possible, and note that digital MEMs microphones have the ADC integrated within the device, and therefore don't require a separate ADC.
Even a simple delay and sum beamforming algorithm is sensitive to microphone channel mismatch. In some cases, a microphone sensitivity mismatch of 6 dB may result in a reduction in wanted signal enhancement of approximately 18 dB. This is a realistic worst-case measure as typical microphone manufacturing tolerances may be in the range +−3 dB.
There are a number of approaches to the microphone sensitivity mismatch problem. One is to specify the microphones have a very tight manufacturing tolerance. Another is to require production line trimming or matching of the microphone tolerance, which has the added disadvantage that sensitivity may drift over time. These two approaches significantly increase production costs, rendering their use impractical or commercially uncompetitive in most instances.